Science Fiction: Read it in the 80s. A man discovers an alien conspiracy to conquer Earth, fights it, is captured and turned into a tank that fights wars on a moon and then rebels and organizes the other 'tanks' to fight against and defeat the evil aliens.

This story I am looking for was in a short story collection which I read at school, many years ago. At dinner in a small hotel or guesthouse, the guests start telling each other stories. One guest claims he can make someone vanish so that every trace of them is erased, and no-one will remember them. He is reluctant to demonstrate his skill, but eventually he is persuaded. The other guests laugh at him because apparently his trick hasn't worked, but it becomes clear to the reader that one of the characters has disappeared.

I have come across other people searching for the same story, but so far without success. It would be wonderful if someone from this forum remembers the story and, better still, knows who wrote it!